This invention relates to ceramic bodies and more particularly to translucent ceramic bodies containing alumina as the main ingredient.
Conventional sintering of alumina involves firing the alumina body at the sintering temperature for extended periods of time. Typically, more than three hours is required to produce a translucent alumina body. Such extended sintering times significantly limit the production rate of translucent alumina bodies. For example, U.S. Pat. No. 3,377,176 to Wolkodoff et al. discloses the production of a high-density alumina body by sintering an alumina compact doped with magnesia and yttria at 1400 to 1550.degree. C. in oxidizing atmosphere, hydrogen or vacuum for four hours. U.S. Pat. No. 3,905,845 to Kobayashi et al. discloses the manufacturing of a polycrystalline translucent alumina body by sintering an alumina compact doped with 0.05-0.5 weight percent of yttria, 0.05-0.5 weight percent of lanthana, and 0.01-0.1 weight percent of magnesia in vacuum, hydrogen, or ammonium decomposed gas at 1600.degree. C. to 1800.degree. C. for 5 hours.
Conventional sintering of alumina further involves hydrogen or vacuum firing. For example, U.S. Pat. No. 3,026,177 to St. Pierre et al. discloses the production of a transparent alumina body by subjecting a compact of alumina to a first firing in hydrogen at 1650.degree. C. to 1750.degree. C. to effect removal of gas-containing pores from the body, and subjecting the body to a second firing in hydrogen at 1800.degree. C. to 2000.degree. C. for not less than 15 minutes to remove additional pores and improve the transparency of the body. U.S. Pat. No. 3,026,210 to Coble discloses the production of a high density alumina body having substantial transparency by forming a mixture of alumina powder and a small but effective amount up to 0.5 weight percent of magnesia powder into a compact, and firing the compact at 1700.degree. C. to 1950.degree. C. in vacuum or hydrogen for 1.5 to 4 hours. U.S. Pat. No. 4,285,732 to Charles et al. discloses the production of a translucent alumina body by sintering an alumina compact doped with magnesia (0.03 to 0.15 wt %) and an additive selected from ZrO.sub.2 (0.002 to 0.07 wt %) and HfO.sub.2 (0.003 to 0.12 wt %) in hydrogen at 1750 to 1950.degree. C. for 3 hours. The cost of such firing is high. Hydrogen is an expensive gas, and use of hydrogen further requires that special furnaces equipped with safety provisions be used. In the case of vacuum firing, high-cost vacuum furnaces are required.
Fast-firing of alumina ceramics has been reported in the literature by Harmer et al. Harmer et al. have described fast firing of alumina ceramics in a zone sintering furnace at 1850.degree. C. for 2 to 15 minutes in air or oxygen atmosphere. Proc. 4th International Meeting on Modern Ceramic Technologies, St. Vincent, Italy 28-31 May 1979, ed. P. Vincenzini, Elsevier Sci. Publ. Co. Amsterdam 1980, p. 155-162. However, the rapid sintering method reported by Harmer et al. produces optically translucent alumina ceramics only in oxygen atmosphere.